For future reference:
Step 1) Get something that knows the move you want.
Step 2) Go to the move deleter in Venesi City and get rid of everything that isn't that move.
Step 3) Go into the Labyrinth Cave with a Smoke Ball, something with Run Away, or a high-speed Ghost-Type to escape any Shadow Tag Linkites until you run into a Duplicat.
Step 4) Switch in the Pokemon with the move you want copied and use a Yellow Flute to pass turns until it uses Transform.
Step 5) Switch over to the Ratsy/Raffiti, or pass turns until the Pokemon is KO'd and make sure the Ratsy/Raffiti has higher speed and/or a Focus Sash if the move and/or Pokemon that the Duplicat transformed into was really strong. Then use Sketch.
Step 6) Don't forget to visit the Move Reminder and re-learn Sketch before repeating the process.
P.S. Consider getting Dusk Balls/Repeat Balls to catch the Duplicat afterward in case it has a few good IVs just in case.
With this you can get almost any combination of moves you can think of (With some exceptions:
https://pokemon-uranium.fandom.com/wiki/Sketch_(move) (Which reminds me, I'm pretty sure trying to Sketch Transform will also fail, but if I
did test that and confirmed it I forgot to put it in the Wiki))
Just keep in mind that you can't re-learn Sketch if the Ratsy/Raffiti is too low a level to have already learned it via leveling up (by breeding it with the move at the start), and that goes for any moves you delete off of mons to leave only one left if those were also bred onto them or can no longer be learned (such as Aerial Ace and Roost being capable of being taught to Tonemy but not its evolution Tofurang). Egg moves can be re-learned from the move reminder at any time, but
only if they are not TM moves and the Pokemon has not evolved from its earliest form.